The use of radiofrequency (RF) and microwave (MW) generators connected to non-cooled electrodes inserted into the tissue of the body so that the signal output from the high frequency (HF) generator ablates the tissue has been used for decades. Both RF and MW generators are considered HF generators here. Use of cooled RF and MW electrode systems have also been in use for decades. Computer graphic systems have been described in use with high frequency ablation systems. The Cosman G4 Radiofrequency generator (Cosman Medical, Inc., Burlington, Mass.) is an example of a modern RF lesion generator that includes a graphic display, and the Cosman G4 brochure printed in 2011 is hereby incorporated by reference herein in its entirety.
A research paper by E. R. Cosman, et al., entitled “Theoretical Aspects of Radio Frequency Lesions in the Dorsal Root Entry Zone,” Neurosurgery, Vol. 15, No. 6, pp. 945-0950 (1984), describes various techniques associated with radio frequency lesions and is hereby incorporated by reference herein in its entirety. Also, research papers by S. N. Goldberg, et al., entitled “Tissue Ablation with Radiofrequency: Effect of Probe Size, Gauge, Duration, and Temperature on Lesion Volume,” Acad. Radiol., Vol. 2, pp. 399-404 (1995), and “Thermal Ablation Therapy for Focal Malignancy,” AJR, Vol. 174, pp. 323-331 (1999), described techniques and considerations relating to tissue ablation with radio frequency energy and are hereby incorporated by reference herein in its entirety. Examples of high-frequency (HF) generators and electrodes are given in the papers of entitled “Theoretical Aspects of Radiofrequency Lesions and the Dorsal Root Entry Zone,” by Cosman, E. R., et al., Neurosurg 15:945-950, 1984; and “Methods of Making Nervous System Lesions,” by Cosman, E. R. and Cosman, B. J. in Wilkins R. H., Rengachary S. S. (eds): Neurosurgery, New York, McGraw-Hill, Vol. Ill. pp. 2490-2498, 1984, and are hereby incorporated by reference herein in their entirety. A paper by D. A. Gervais, et al., entitled “Radiofrequency Ablation of Renal Cell Carcinoma: early Clinical Experience,” Radiology, Vol. 217, No. 2, pp. 665-672 (2000), describes using a rigid tissue perforating and penetrating electrode that has a sharpened tip to self-penetrate the skin and tissue of the patient for the ablation of kidney tumors, and this paper is hereby incorporated by reference herein in its entirety.
United States patents by E. R. Cosman and W. J. Rittman, III, entitled “Cool-Tip Electrode Thermal Surgery System,” U.S. Pat. No. 6,506,189 B1, date of patent Jan. 14, 2003; “Cluster Ablation Electrode System,” U.S. Pat. No. 6,530,922 B1, date of patent Mar. 11, 2003; and “Cool-Tip Radiofrequency Thermosurgery Electrode System For Tumor Ablation”, U.S. Pat. No. 6,575,969 B1, date of patent Jun. 10, 2003 describe systems and methods related to tissue ablation with radiofrequency energy, generators, and internally-cooled RF electrodes, and they are hereby incorporated by reference herein in their entirety. One electrode system described in these patents comprises an electrode with an insulated shaft except for a fixed uninsulated tip exposure of an uninsulated exposed length, the electrode being internally cooled so that the uninsulated exposed tip is cooled. The electrode shaft is a rigid and self tissue piercing with a sharp pointed distal tip on the electrode shaft. This is essentially the configuration of cooled electrode offered by the Radionics Cool-Tip Electrode System (Radionics, Inc., Burlington Mass.) and the Valley Lab Cool-Tip Electrode System (Valley lab, Inc., Boulder Colo.). One cooled-RF electrode shown in patent '189 includes an extension tip including a temperature sensor at its distal end. In a patent by Mark Leung, et al., entitled “Electrosurgical Tissue Treatment Method”, U.S. Pat. No. 7,294,127 B2, date of patent: Nov. 13, 2007, a cooled RF electrode is shown. These patents are hereby incorporated by reference herein in its entirety. The cooled RF system manufactured by Baylis Medical Company (Canada) includes an RF generator maintains the temperature of an internally-cooled RF electrode substantially below the tissue boiling point, wherein the electrode includes a temperature sensor positioned in an extension tip at the distal end of the electrode shaft.
United States patent applications by E. R. Cosman Jr and E. R. Cosman Sr. “Cool RF Electrode” application Ser. No. 13/153,696, “Cool RF Electrode” application Ser. No. 14/072,588, and “Cool RF Electrode” application Ser. No. 14/076,113, describe systems and methods related to tissue ablation with radiofrequency energy, generators, internally-cooled RF electrodes, and RF cannulae, and they are hereby incorporated by reference herein in their entirety.
The following patents describe microwave tissue ablation devices and are herein incorporated by reference in their entireties: U.S. Pat. Nos. 4,641,649, 5,246,438, 5,405,346, 5,314,466, 5,800,494, 5,957,969, 6,471,696, 6,878,147, and 6,962,586. Microwave energy is typically delivered to tissue by application of a high frequency electromagnetic signal to an elongated antenna probe placed in bodily tissue, without the use of a reference ground pad, wherein the frequency is in the range 800 MHz to 6 GHz, for example. Currently the frequencies that are approved by the U.S. Food and Drug Administration for clinical work are 915 MHz and 2.45 GHz. Examples of companies with MW ablation systems includes Evident, Covidien, Mansfield, Mass.; MicrothermX, BSD Medical, Salt Lake City, Utah; Avecure, Medwaves, San Diego, Calif.; Certus 140, Neuwave, Madison, Wis.; Amica, Hospital Service, Rome, Italy; and Acculis MTA, Microsulis, Hampshire, England.
Examples of non-cooled electrode systems are in the product lines of the companies Cosman Medical, NeuroTherm, Radionics, ValleyLab, Baylis, Kimberly Clark, Stryker, and Diros Medical. Examples of non-cooled electrode systems with computer graphic display are given in the product lines of Cosman Medical, NeuroTherm, Stryker, Baylis, Kimberly Clark, and Diros. Examples of cooled high-frequency electrode system are shown in the product lines of Covidian, HS Medical, Boston Scientific, and Kimberly Clark. Generator systems for RF nerve ablation are included in the product lines of Cosman Medical, NeuroTherm, Radionics, ValleyLab, Baylis, Kimberly Clark, Stryker, and Diros Medical, and they produce maximum power 50 Watts, attach to up to four electrodes, and attach to one ground pad. Generator systems for RF tumor ablation are included in the product lines of Covidian, HS Medical, and Boston Scientific, and they produce maximum power 200 or 250 Watts, attach to up to three electrodes, and attach to up to four ground pads. One limitation of these RF generators is that they do not provide real-time graphic display of impedance as function of time. One limitation of these RF generators is that they do not provide real-time graphic display of current as function of time. One limitation of these RF generators is that they do not provide real-time graphic display of both current and impedance as function of time. One limitation of these generators is that they do not provide real-time graphic display of both impedance and the generator output level as function of time. One limitation of these generators is that they do not provide real-time graphic display of impedance, current, and temperature as function of time. One limitation of these generators is that they do not provide real-time graphic display any two of the following list: current, voltage, power, impedance, or mathematical functions with these parameters as arguments. One limitation of these RF generators that are configured for cooled RF tissue ablation by means of an automated impedance-driven pulsing process is that they do not provide a graphic display of any measured parameter as a function of time axis. While some generator systems in the prior art have allowed for export of generator readings which could be used to plot parameters on the same time axis after the ablation procedure, this does not provide instant feedback that would allow a clinician to monitor the ablation process as it progresses and to make adjustments if necessary. This advantage of real-time graphical doctor feedback has special importance for cooled-RF tissue ablation, because a single RF electrode can deliver very high current and heating power to influence tissue several centimeters away from that RF electrode (in contrast to typical non-cooled monopolar and bipolar surgical coagulation, for example), and because the amplitude and timing of impedance and output-level signals can indicate irregularities that can arise from heating over a large volume of tissue that may include multiple tissue types within different electrical and thermal characteristics.
U.S. Pat. No. 5,233,515 A, date of patent Aug. 3, 1993, entitled “Real-time graphic display of heat lesioning parameters in a clinical lesion generator system”, by E. R. Cosman and W. J. Rittman, 111, describes a system for non-cooled RF ablation wherein temperature and impedance are plotted on the same time axis, and is hereby incorporated by reference herein in its entirety. One limitation of '515 is that no parameter of the RF generator signal output level (eg voltage, current, power) is plotted as a function of a time. Another limitation of '515 is that the RF generator signal output level (eg voltage, current, power) is not plotted on the same time axis as the impedance. Another limitation of '515 is that the system does not pertain to RF ablation using an internally-cooled RF electrode. Another limitation of '515 is that it does not pertain to automated methods of repeatedly pulsing the RF signal output.
U.S. Pat. No. 6,241,725, date of patent Jun. 5, 2001, entitled “High frequency thermal ablation of cancerous tumors and functional targets with image data assistance”, by E. R. Cosman, describes a system for planning non-cooled RF ablation using temperature monitoring and control, including plots of various RF parameters. One limitation of '725 is that the system does not pertain to RF ablation using an internally-cooled RF electrode. Another limitation of '725 is that it does not pertain to automated methods for impedance-feedback control of a cooled RF electrode. Another limitation of '725 is that it does not pertain to automated methods comprising repeated pulsing the RF signal output.
Control methods for cooled RF electrodes exist in the prior art wherein the generator signal alternates between a high range and a low range in response to impedance spikes indicative of tissue boiling, which produces high-impedance gaseous vapor bubbles around the electrode active tip. The paper entitled “Percutaneous radiofrequency tissue ablation: optimization of pulsed-RF technique to increase coagulation necrosis” by S. N. Goldberg et al. (J Vasc Interv Radiol 1999; 10(7):907-916) and the paper entitled “High-Power Generator for Radiofrequency Ablation: Larger Electrodes and Pulsing Algorithms in Bovine ex Vivo and Porcine in Vivo Settings” by S. A. Solazzo et al. (Radiology 2007; 242(3):743-750) are hereby incorporated by reference in full and describe pulsing processes wherein the RF signal is delivered at a high, constant-current level until impedance rises above a threshold indicative of boiling (the “up time”); then the RF signal level is substantially reduced for a predetermined duration (the “down time”); then the RF output level is returned either to the previous high constant-current level or to a constant-current level 100 mA below the previous high constant-current level depending on whether the duration of the previous “up time” was above or below a threshold, respectively; and then the cycle of up times and down times repeats throughout the ablation process. Therefore, after an initial rapid ramp (in Solazzo, the initial predetermined current level is achieved in at most 30 seconds, at a rate of 67 mA/sec or 134 mA/sec) to bring the RF output to its initial set level, the level of successive constant-amplitude RF pulses monotonically decreases during the ablation process. One challenge in cooled RF control by means of impedance-based pulsing methods is the selection of a target radiofrequency signal level, eg current, that can produce a stable ablation process, because the maximum signal level that particular tissue can carry without overheating and limiting ablation size, can vary across tissue types, patients, and bodily locations. One limitation of the prior art in impedance-based pulsing methods for cooled RF ablation control is that the initial output level ramp is too fast to discriminate the maximum output level ramp that the tissue can stably carry during the ablation process. One limitation of the prior art in impedance-based pulsing methods for cooled RF ablation control is that the output level, namely the current level, does not increase from the initial set value during the ablation process. One limitation of the prior art in impedance-based RF pulsing methods is that if the output level is set low to avoid the risk of overheating the tissue, then the maximum ablation size may not achieved, or the maximum lesion size may be not achieved as efficiently as possible. In one aspect, the present invention seeks to overcome these limitations by means of an RF pulsing method that can both increase and decrease the generator output level during up times, eg current, in response to measured ablation parameters. Another limitation of the prior art in pulsing methods for cooled RF ablation is that the “down times” (that is, the inter-pulse cooling times) do not vary during an ablation session. The down times do not vary either in accordance with a predetermined schedule or in response to a measured parameter. This is an important limitation because the extent of the region of boiling tissue bubbles can change during the ablation process, and/or because the heat distribution around the bubble zone changes the rate of dissipation of the bubbles, and/or because a predetermined down-time duration may not be well matched to the every ablation scenario, leading to a situation where more or less inter-pulse cooling time is required for optimal dissipation of vapor bubbles in the tissue. In one aspect, the present invention seeks to overcome these limitations by means of an RF pulsing method wherein the down-time durations (ie the cooling time in between pulses) can vary during the ablation process. In some embodiments of the present invention, the variation of down times can include a component that is predetermined, as in the example of a predetermined schedule of increase in the down-times durations. In some embodiments of the present invention, the variation of down times can include a component that is influenced by one or more measured parameters during the ablation process. In various embodiments of the present invention, the variation of down times can either strictly increase, strictly decrease, or both increase and decrease. Another limitation of prior systems for RF tissue ablation is that they do not plot both the impedance and the generator output level (eg voltage, current, or power) on the same time axis in real time. Another limitation in the prior art is that prior systems for RF tissue ablation do not include a plot of two or more of the parameters impedance, voltage, current, and power. Another limitation in the prior art is that prior systems for RF tissue ablation do not include both an automatic method for output-level pulsing (eg for cooled RF tissue ablation) and a real-time graphical plot of a parameter of the generator output (eg voltage, current or power, impedance) as a function of time. Several aspects of the present invention seek to overcome these limitations.
In U.S. Pat. Nos. 8,152,801 and 8,357,151 by Goldberg and Young, an RF pulsing method for non-cooled RF ablation is proposed wherein the level of successive constant-amplitude RF pulses monotonically decreases during the ablation process in response to impedance variations indicative of tissue moisture content.
The use of radiofrequency (RF) generators and electrodes to be applied to tissue for pain relief or functional modification is well known. Related information is given in the paper by Cosman E R and Cosman B J, “Methods of Making Nervous System Lesions”, in Wilkins R H, Rengachary S (eds.); Neurosurgery, New York, McGraw Hill, Vol. 3, 2490-2498; and is hereby incorporated by reference in its entirety. Related information is given in the book chapter by Cosman E R Sr and Cosman E R Jr. entitled “Radiofrequency Lesions.”, in Andres M. Lozano, Philip L. Gildenberg, and Ronald R. Tasker, eds., Textbook of Stereotactic and Functional Neurosurgery (2nd Edition), 2009, and is hereby incorporated by reference in its entirety. For example, the RFG-3C plus RF lesion generator of Radionics, Inc., Burlington, Mass. and its associated electrodes enable electrode placement of the electrode near target tissue and heating of the target tissue by RF power dissipation of the RF signal output in the target tissue. For example, the G4 generator of Cosman Medical, Inc., Burlington, Mass. and its associated electrodes such as the Cosman CSK, and cannula such as the Cosman CC and RFK cannula, enable electrode placement of the electrode near target tissue and heating of the target tissue by RF power dissipation of the RF signal output in the target tissue. Temperature monitoring of the target tissue by a temperature sensor in the electrode can control the process. Heat lesions with target tissue temperatures of 60 to 95 degrees Celsius are common. Tissue dies by heating above about 45 degrees Celsius, so this process produces the RF heat lesion. For pain management, RF generator output is also applied to nerves using a type of pulsed RF method, whereby RF output is applied to tissue intermittently such that tissue is exposed to high electrical fields and average tissue temperatures are lower, for example 42 degrees Celsius or less; this is different from the RF pulsing methods for tissue ablation, such as that described in Goldberg et al. (1999) and in embodiments of the present invention, wherein the clinical objective is to heat large volumes of tissue surrounding the active tip to destructive temperatures, including temperatures that induce tissue boiling. High temperatures in pain management pulsed RF are undesired and are only sometimes present over very small regions (eg less than 0.33 mm in radius) near point of high curvature on active electrode tip, as described in an article by E. R. Cosman Jr. and E. R. Cosman Sr. entitled “Electric and thermal field effects in tissue around radiofrequency electrodes” (Pain Medicine 2005; 6(6): 405-424) which is hereby incorporated by reference in full. This is different the temperature profile produced by RF tissue ablation wherein approximately ellipsoidal high-temperature isotherms surround the electrode active tip and spread several millimeters or several centimeters from the active tip, thereby heating a substantially portion of the tissue in contact with the electrode active tip to a destructive temperature. The pain-management pulsed RF method is applied with pulses of RF that have duration in the range less than 50 milliseconds, with the RF level at zero in between pulses, and with pulse repetition rates of 1 to 10 Hz, so that the duration of each period wherein the RF is on is less than 50 milliseconds, and the duration of each period wherein the RF is off is less than 1000 milliseconds; this is different from RF pulsing methods for control of tissue ablation electrodes, such as that described for cooled RF in Goldberg et al. (1999) and in embodiments of the present invention, wherein the duration of each period in which RF is applied at a high level configured to substantially heat the tissue typically greater than 10 seconds and can be as long as the total time of the ablation process, and wherein the duration of each period wherein the RF is applied at a low level configured to allow tissue cooling is typically between 5 and 50 seconds. The pain-management pulsed RF method either adjusts the signal parameters pulse amplitude, pulse rate, and pulse width in response to a measured temperature; or fixes these values; and does not terminate and initiate RF pulses in response to either indications of tissue boiling, indications of tissue cooling, the expected duration of tissue cooling between RF pulses, or the value or variations of a measured impedance, current, voltage, or power. This is different from RF pulsing methods for control of tissue ablation, such as that described for cooled RF in embodiments of the present invention and in Goldberg et al. (1999), wherein the up times are terminated by a rise in impedance, and the down times have a duration configured to allow for the dissipation of high-temperature and high-impedance gas formed around the active electrode tip. The RF pulses in the pain-management method of pulsed RF are configured to reduce tissue heating; whereas the RF pulses in tissue-ablation pulsed RF methods, such as those presented in the present invention, are configured to maximize tissue heating. The durations of low signal level between RF pulses in the pain-management method of pulsed RF are not configured to allow for dissipation of gas bubbles distributed around the electrode active tip; whereas the durations of low signal level between RF pulses in the pulsed-RF methods of the present invention are configured to allow for dissipation of large gas bubbles distributed around the electrode active tip. The level of RF pulses in pain-management pulsed RF are not configured to increase a heat lesion size; whereas the level of RF delivered during the on periods of embodiments of the present invention are configured to increase the size of a heat ablation volume.
A significant difference between non-cooled electrode systems and cooled electrode systems is in the means of controlling the ablation process. For non-cooled electrode systems, the maximum tissue temperature is substantially at or near the surface of the electrode, so measuring the electrode temperature by a temperature sensor in the electrode, one can directly control the ablation process. For a given electrode temperature, size of electrode, and time of heating, you can predict reliably ablation size as described in the papers entitled “Theoretical Aspects of Radiofrequency Lesions and the Dorsal Root Entry Zone,” by Cosman, E. R., et al., Neurosurg 15:945-950, 1984, and “Bipolar Radiofrequency Lesion Geometry: Implications for Palisade Treatment of Sacroiliac Joint Pain.” By E. R. Cosman Jr and C. D. Gonzalez, Pain Practice 2011; 11(1): 3-22, which are herein incorporated by reference in their entireties. For non-cooled electrodes it is also possible to prevent the instability point of boiling of tissue, explosive gas formation, and charring of tissue by direct control of the HF generator signal output so the electrode temperature does not exceed 100° C. as described a monograph entitled “Guide to Radio Frequency Lesion Generation in Neurosurgery” by B. J. Cosman and E. R. Cosman, Radionics, Burlington, Mass., 1974.
A cooled-electrode HF ablation system differs from a non-cooled-electrode HF ablation system in that the maximum tissue temperature is at a distance from the electrode. The maximum tissue temperature around a cooled electrode occurs in a zone around the electrode tip, but at a distance from the electrode tip. The electrode is cooled so the electrode temperature is not typically a direct measure of the maximum tissue temperature, unlike non-cooled electrode systems wherein the maximum tissue temperature can be measured almost directly by means of the non-cooled-electrode's temperature sensor. Cooled-electrode HF ablation using an satellite temperature sensor, such as an extension tip containing a temperature sensor, can be temperature-controlled to prevent tissue boiling; this is different from cooled-electrode HF ablation in which the tissue temperature is not monitored (such as the case wherein the electrode does not contain a temperature sensor, or the case wherein the electrode temperature sensor is within the flow of coolant within the electrode) and the electrode is allowed to raise tissue temperatures into the boiling range.
The use of RF energy in neural tissue for the treatment of pain and functional disorders is well known. A typical nerve ablation protocol includes a first step in which one or more nerve stimulation signal is applied to an RF electrode for guidance of that electrode, and the a second step in which RF energy is applied to the RF electrode to ablate tissue near the electrode active tip. Typical nerve stimulation signals include biphasic electrical pulses delivered at a rate of up to 200 Hz, typically 50 Hz for sensory nerve stimulation and 2 Hz for motor nerve stimulation. Another well-known clinical use of high-frequency energy is the ablation of large tumors; this requires putting large amounts of power from the electrode into the tissue. This will cause the zone of maximum temperature to exceed 100 degrees C. That will cause the tissue to boil and bubbles to form in the zone of maximum temperature. This can be a rapidly explosive process and an unstable process. For tumor ablation performed using a cooled electrode, the temperature measured inside the cooled electrode is not a direct indication of the zone of instability. However, the instability is reflected in other signal output parameters, including, for example, the signal output impedance, power, current, and voltage. The above background references do not teach how to control the cooled HF electrode process when the generator signal output is increased so that the process is pushed into the region on of instability, nor do they show or teach how to maintain and monitor an ablation process that is held close to the instability point for the duration of the process.
U.S. Pat. No. 7,736,357 by Lee et al. (hereinafter “Lee”) presents a radiofrequency ablation system wherein RF current from an ablation electrode is switched between two or more ground pads in a repeating sequence wherein only one ground pad is active at a time. One limitation of the prior art in Lee is that it does not provide for simultaneous activation of multiple ground pads at the same time during a switching sequence. Another limitation of Lee is that the peak current at each ground pad is identical and equal to the total current delivered to one or more ablation electrodes. This peak current can be very high, and can limit the total current that can be delivered by the ablation electrodes. Another limitation of the prior art in Lee is that the sequence of switch states is predetermined, alternating sequentially among a number of ground pads. Another limitation of the prior art in Lee is that the sequence of switch states is not based on a measurement of a ground pad parameter. Another limitation of the prior art in Lee is that a sequential ground-pad switching sequence does not generally minimize tissue heating for each ground pad relative to other switching sequences. Another limitation of the prior art in Lee is that a sequential ground-pad switching sequence does not maximize the total ablation current that a configuration of ground pads can carry, across all possible ground-pad switching sequences. Another limitation of the prior art in Lee is that it does not provide a switching method that can control both the total rate of heating in tissue adjacent to two or more ground pads, and the rate of heating in the tissue region adjacent to each one of the said two or more ground pads. In the prior art in Lee, for a given total current I delivered N ground pads by the ablation electrodes, the total average heating power delivered to tissue in contact with the two or more ground pads is proportional to (I2*t1/t)+ . . . +(I2*tN/t)I2, where ti is the duration of i-th phase of the switching sequence in which only the i-th pad is connected and carrying current, and where t=t1+ . . . +tN is the total duration of one cycle of the switching sequence. Therefore, while the average heating power delivered to the i-th pad (which is proportional to the RMS current I2*t1/t delivered to the pad over the cycle period) can be controlled by adjustment of the duration ti, the total average heating power is invariant variations in the timing of the switching pattern. Another limitation of Lee is that a cyclic switching sequence does not maximize the total current which can be carried by a set of ground pads, where the RMS current each ground pad is held below a safety limit. Another limitation is that the system of Lee does not reduce the number of switching transitions. Another limitation is that the system of Lee does not provide for both switching and independent current-monitoring for each pad.
The papers “Sequential Activation of a Segmented Ground Pad Reduces Skin Heating During Radiofrequency Tumor Ablation: Optimization via Computational Models”, IEEE Trans Biomed Eng. 2008 July; 55(7): 1881-9 by D. J. Schutt and D. Haemmerich; “Sequential activation of ground pads reduces skin heating during radiofrequency ablation: Initial in vivo porcine results” Conf Proc IEEE Eng Med Biol Soc. 2009; 1:4287-4290 by D. J. Schutt et al.; and “Sequential Activation of Ground Pads Reduces Skin Heating During Radiofrequency Tumor Ablation: In Vivo Porcine Results” IEEE Trans Biomed Eng 2010 March; 57(3):746-753 by D. J. Schutt et al. describe switching of power-regulated RF output to three ground pads in known positions relative to each other, wherein each pad includes a temperature sensor, wherein the switching sequence maintains the temperatures at a set level, and wherein the switching produces repeated cycles of the sequence: (1) proximal, middle, and distal ground pads activated; (2) middle and distal ground pads activated; and (3) only distal ground pad activated. One limitation the papers of Schutt et al. is that they do not provide for control the current carried by any one of the ground pads in relative to a target current value or a maximum current value. One limitation the papers of Schutt et al. is that they do not provide for automatic control of the root-mean-squared (RMS) current at each pad over each switching cycle. Another limitation of the papers by Schutt et al. is that integration of temperature sensors into the ground pads is required. One limitation preventing ground pad burns by temperature monitoring is that the temperature sensor may not directly or reliably measure the temperature of heated tissue, for example, in the case where the ground pad is not fully adhered to the skin. Another limitation of the papers by Schutt et al. is that the relative position of the ground pad was known ahead of time and used to set up the switching process manually, rather than automatically by means of a controller based on a measured parameter of a ground pad. Another limitation of the papers by Schutt et al., is that neither the ground-pad switch states (as reflected by the identities of the connected ground pads and disconnected ground pads) in the sequence, nor the order of the switch states in the sequence, was determined by an automatic controller using a measured ground-pad parameter. Another limitation of the papers by Schutt et al., is that the identity and order of switch states is predetermined. Another limitation of the papers by Schutt et al. is that they do not provide for both switching and current monitoring at each pad individually. Another limitation of the papers by Schutt et al. is that ground pad switch was used in conjunction with an ablation electrode output that was set to a constant power, which can lead to variable current density around the electrode active tip as ground pads are connected and disconnected, and thus potentially leading to inconsistent lesion sizes at the ablation site. Another limitation of the papers by Schutt et al. is that they do not provide for both switching and independent current-monitoring for each pad.
U.S. Pat. No. 7,566,332 by Jarrard and Behl presents a radiofrequency ablation system wherein the RF current flowing to each of two or more ground pads from an ablation electrode is balanced by adjusting a limited amount added resistance between the ground pad and the RF power supply. One limitation of adding resistance between a ground pad and the power supply is that generated electrical energy is dissipated in the resistance and does not heat the target tissue, thereby limiting the maximum heating power that the generator can produce. One limitation of the prior art in '322 is that the variety of ground pad configurations to which the system can adapt is limited by the limited amount of resistance that is added to each ground pad line during an ablation procedure.
Another limitation of the prior art is that RF ablation systems configured for nerve ablation do not include means for connection and monitoring of multiple ground pads. This is a significant limitation for energizing multiple nerve ablation electrodes in a single patient at the same time, which can produce high currents in excess of current capacity of a single typical nerve-ablation ground pad.
Another limitation of the prior art on ground pad switching in Lee and Schutt et al. is that they do not provide for the prevention or reduction of electrical stimulation of excitable tissue that can occur due to transient direct-current signals that can arise when a switch opens or closes. Undesired stimulation of excitable tissue can occur at a site remote of the ground pads and ablation electrodes. Undesired stimulation of nerves can occur, and be disturbing to the patient, due to high electric field strengths near the active tip of an RF ablation electrode and transients produced by connection or disconnecting a ground pad from the source of the RF ablation signal.
U.S. Pat. Nos. 6,575,969 and 6,506,189 by Rittman and Cosman, and U.S. Pat. No. 6,241,725 by E. R. Cosman relate to the use of ultrasound imaging data for tissue ablation. In the prior art, ultrasound imaging apparatuses are physically separate from HF ablation generators, and there is no connection between an ultrasound imaging device and a HF ablation generator for unified control of both devices, image and parameter display, and procedure documentation. One limitation the prior art in ultrasound image guidance for tissue ablation is that the physician must use a two sets of controls to operate both the ultrasound imaging device and the HF ablation generator. In one example, the absence of a single user interface for control and monitoring of both ultrasound imaging and ablation readings is a limitation for ultrasound-guided cooled RF tumor ablation using an RF pulsing process that repeatedly induces tissue boiling, because the ultrasound images and the RF generator readings both provide rich information to the operating physician who, as does the correlation of ultrasound features and features of RF generator readings (eg echogenic bubble formation and variations in RF impedance. Another limitation of the prior art is the absence of automated influence of the ablation process using ultrasound imaging data as an input.
The use of RF energy in neural tissue for the treatment of pain and functional disorders is well known. A typical nerve ablation protocol includes a first step in which one or more nerve-stimulation signal configured to induce repeated nerve firing, is applied to an RF electrode for guidance of that electrode to a target position near a near, a second step in which a fluid anesthetic is injected to prevent perception of pain during the nerve ablation, and a third step in which RF energy is applied to the RF electrode to ablate tissue near the electrode active tip. Typical nerve stimulation signals include biphasic electrical pulses delivered at a rate of up to 200 Hz, typically 50 Hz for sensory nerve stimulation, and typically 2 Hz for motor nerve stimulation. One limitation of the prior art is that RF and nerve-stimulation signals are not applied at the same time to a single peripheral nerve. One limitation of the prior art is that RF and nerve-stimulation signals are not applied at the same time to a single peripheral nerve, and firing in that nerve is not monitored at the same time. One limitation of the prior art is that RF and nerve-stimulation signals are not repeatedly interleaved to provide for nerve stimulation throughout an RF ablation process. One limitation of the prior art is the response of a nerve to a stimulation single is not used as a stopping criteria for an RF nerve ablation. Another limitation of the prior art is that an RF generator configured for nerve ablation does not produce nerve stimulation signals that are configured to electrically block the transmission of action potentials within a nerve, such as a high-frequency block signal.
The present invention overcomes the stated disadvantages and other limitations of the prior art.